Tonometry system for determining blood pressure

Claims

I claim:

1. For use in a system for noninvasively determining the intra-arterial blood pressure of a patient, a tissue contact stress sensing apparatus comprising:

a wafer having a continuous diaphragm for placing against a patient's tissue which covers an underlying artery, said diaphragm adapted to be deformed in response to stress in said tissue caused by the intra-arterial blood pressure of said artery,

semiconductor assembly means placed in close proximity to and spaced apart from said continuous diaphragm for directly,

irradiating said diaphragm with electromagnetic radiation,

receiving a portion of said electromagnetic radiation reflected from said continuous diaphragm, and

whereby the quantity of electromagnetic radiation received by said semiconductor assembly means is a function of the displacement of said continuous diaphragm in response to said tissue stress caused by said intra-arterial blood pressure of said artery.

2. The tissue contact stress sensing apparatus of claim 1 wherein said diaphragm is comprised of silicon.

3. The tissue contact stress sensing apparatus of claim 2, wherein said diaphragm is comprised of single crystal silicon.

4. The tissue contact stress sensing apparatus of claim 2, wherein said wafer is comprised of silicon and has a longitudinal trough therein and, wherein the bottom of said trough forms one side of said diaphragm.

5. The tissue contact stress sensing apparatus of claim 4, wherein said longitudinal trough has a cross-sectional profile generally resembling a tetragonal-pyramidal geometry.

6. The tissue contact stress sensing apparatus of claim 4, wherein said trough has a generally planar bottom.

7. The tissue contact stress sensing apparatus of claim 1, wherein said wafer has a longitudinal trough therein and, wherein the bottom of said trough forms one side of said diaphragm, and wherein said trough bottom is coated with a reflective material for reflecting the electromagnetic radiation irradiated by said semiconductor assembly means.

8. The tissue contact stress sensing apparatus of claim 7, wherein said trough bottom is generally 6.5.times.10.sup.-6 meters thick.

9. The tissue contact stress sensing apparatus of claim 7, wherein said trough bottom is coated with metal.

10. The tissue contact stress sensing apparatus of claim 9, wherein said metal is selected from the group consisting of gold or aluminum.

11. The tissue contact stress sensing apparatus of claim 9, wherein said metal coating is generally 600 angstroms thick.

12. The tissue contact stress sensing apparatus of claim 1, wherein said diaphragm has an effective stiffness which is sufficient to prevent significant distortion of the stress information contained within the received electromagnetic radiation.

13. The tissue contact stress sensing apparatus of claim 1, wherein said continuous diaphragm has a strain rate of generally 0.24 micro-inch/mmHg.

14. The tissue contact stress sensing apparatus of claim 1, wherein said diaphragm has two opposing longitudinal sides spaced generally 0.020 inches apart.

15. The tissue contact stress sensing apparatus of claim 1, wherein said wafer has a generally rectangular face for placing against said tissue overlying said artery, said face having a first and second set of opposing parallel sides, wherein said first set of opposing sides of said rectangular face are generally 0.577 inches apart, and wherein said diaphragm is bounded by at least two generally parallel sides spaced generally 0.425 inches apart.

16. The tissue contact stress sensing apparatus of claim 15, wherein said second set of opposing parallel sides of said face are generally 0.200 inches apart.

17. The tissue contact stress sensing apparatus of claim 1 further including a spacer element disposed between said wafer and said semiconductor assembly means and wherein said spacer element, diaphragm, wafer and semiconductor assembly means all have a substantially similar thermal expansion coefficient for minimizing thermally induced drift and offset errors.

18. The tissue contact stress sensing apparatus of claim 17, wherein said spacing element is comprised of silicon nitride.

19. The tissue contact stress sensing apparatus of claim 1, wherein said semiconductor assembly means is comprised of a plurality of emitters for irradiating said diaphragm with electromagnetic radiation, and a plurality of detectors each having an output, each detector receiving a portion of said electromagnetic radiation and transducing said received radiation into a respectively associated electronic current output signal.

20. The tissue contact stress sensing apparatus of claim 19, wherein said continuous diaphragm is generally planar having a rectangular boundary, said boundary having an opposing pair of long sides and an opposing pair of short sides.

21. The tissue contact stress sensing apparatus of claim 20, wherein said plurality of emitters are arranged generally in a row which is generally parallel to and spaced apart from one of said long sides of said diaphragm, and wherein said detectors are arranged generally in a row separate from said row of emitters, and generally parallel to and spaced apart from one of said long sides of said diaphragm.

22. The tissue contact stress sensing apparatus of claim 21, wherein said emitters are generally equally spaced apart from one another and said detectors are generally equally spaced apart from one another, and wherein said row of emitters is juxtaposed to said row of detectors such that any two adjacent detectors in said row of detectors lie equidistant from at least one common emitter in said row of emitters.

23. The tissue contact stress sensing apparatus of claim 19, wherein each emitter in said plurality of emitters irradiates electromagnetic radiation in a beam field which when projected onto said diaphragm, overlaps the projected beam field of its closest adjacent neighbors.

24. The tissue contact stress sensing apparatus of claim 19, wherein said electromagnetic radiation is selected from the group consisting of visible, infrared and ultraviolet light.

25. The tissue contact stress sensing apparatus of claim 19, further comprising converter means, coupled to the outputs of said detectors, for converting said electric current signal output by each detector into an electric voltage signal.

26. The tissue contact stress sensing apparatus of claim 25, further comprising multiplexing means and central processing means, said multiplexing means coupled to said converter means for selecting one of said converted electronic signals and transferring said signal to said central processing means for analysis.

27. The tissue contact stress sensing apparatus of claim 1, wherein said semiconductor assembly means includes a portion for irradiating electromagnetic radiation onto and detecting electromagnetic radiation reflected from a reference surface, said reference surface spaced apart from said diaphragm.

28. The tissue contact stress sensing apparatus of claim 27, wherein said semiconductor assembly means and said portion of said semiconductor assembly means are fabricated from substantially the same materials and reside in close proximity to each other.

29. The tissue contact stress sensing apparatus of claim 28, wherein said electromagnetic radiation is selected from the group consisting of visible, infrared and ultraviolet light.

30. For use in a system for noninvasively determining the intra-arterial blood pressure of a patient, a tissue contact stress sensor for generating a tissue contact stress signal indicative of said intra-arterial blood pressure and a correction signal for compensating for errors in said tissue contact stress signal caused by temperature drift and aging of said sensor, said system comprising:

a wafer having a continuous diaphragm and a nonresponsive portion, said continuous diaphragm for placing against a patient's tissue which covers an underlying artery, said diaphragm adapted to be deformed in response to said intra-arterial blood pressure of said artery,

semiconductor assembly means spaced apart from and placed in close proximity to said continuous diaphragm for directly,

irradiating said diaphragm with electromagnetic radiation,

receiving a portion of said electromagnetic radiation reflected from said continuous diaphragm,

converting said received radiation into a tissue contact stress signal which represents blood pressure data whereby the quantity of electromagnetic radiation which is received by said semiconductor assembly means is a function of the displacement experienced by said continuous diaphragm in response to said intra-arterial blood pressure of said artery,

spacing structure coupled to said nonresponsive portion of said wafer and said semiconductor assembly means, said spacing structure fixing said separation between said semiconductor assembly means and said wafer,

a portion of said semiconductor assembly means, spaced apart from and placed in close proximity to said nonresponsive portion of said wafer for directly,

irradiating said nonresponsive portion of said wafer with electromagnetic radiation,

receiving a portion of said electromagnetic radiation reflected from said nonresponsive portion of said wafer,

converting said received radiation into a correction signal which represents reference data which is indicative of at least one reference factor, and

whereby any change in the radiation received by said portion of said semiconductor assembly means is attributed to at least one of said reference factors and whereby said correction signal is combined with said tissue contact stress signal in a manner which minimizes the dependence of said blood pressure data on at least one of said reference factors.

31. The tissue contact stress sensor of claim 30, wherein said reference factors include temperature and aging of said wafer, semiconductor assembly means and spacing structure.

32. The tissue contact stress sensor of claim 30, wherein said semiconductor assembly means and said portion of said semiconductor assembly means are fabricated from substantially the same materials and at substantially the same time and said materials reside in close proximity to each other.

33. The tissue contact stress sensor of claim 30, wherein said diaphragm is comprised of single crystal silicon.

34. The tissue contact stress sensor of claim 33, wherein said spacing structure is comprised of silicon nitride.

35. The tissue contact stress sensor of claim 30, wherein said electromagnetic radiation is selected from the group consisting of visible, infrared, and ultraviolet light.

36. The tissue contact stress sensor of claim 30, wherein said diaphragm is comprised of a silicon base having a longitudinal trough therein.

37. The tissue contact stress sensor of claim 36, wherein said trough has a generally planar bottom.

38. The tissue contact stress sensor of claim 37, wherein said trough bottom is generally 6.5.times.10.sup.-6 meters thick.

39. A method for correcting errors in the output signal of a tissue contact stress sensor, said errors caused by the effects of aging and environmental factors on said sensor, said tissue contact stress sensor of the type having an element for placing against a patient's tissue covering an artery of interest, said element responsive to tissue stress, comprising the steps of:

constructing a tissue contact stress sensor and a reference sensor from substantially identical materials at substantially the same time,

adapting said tissue contact stress sensor to be responsive to said element,

adapting said reference sensor to be responsive to a fixed reference source which does not vary with said tissue contact stress measured by said tissue contact stress sensor, whereby an output signal of said reference sensor only changes as a function of said reference sensor aging, temperature and environmental factors,

adapting said tissue contact stress sensor and said reference sensor to share the same environment so as to be equally influenced by aging and environmental factors, and

combining the output signals generated by said tissue contact stress sensor and said reference sensor in a way which removes said influence of aging and environmental factors from said output of said tissue contact stress sensor.

40. For use in a system for noninvasively determining the intra-arterial blood pressure of a patient, a miniaturized tissue contact stress sensing apparatus comprising:

a silicon wafer having a nonresponsive portion and a continuous silicon diaphragm portion, said continuous silicon diaphragm portion for placing against a patient's tissue which covers an underlying artery, said diaphragm adapted to be responsive to stress in said tissue caused by blood pressure pulsations in said underlying artery,

integrated circuit means placed in close proximity to and spaced apart from said continuous diaphragm for directly,

irradiating said diaphragm with electromagnetic radiation,

receiving a portion of said electromagnetic radiation reflected from said continuous diaphragm, and

whereby the quantity of electromagnetic radiation received by said integrated circuit means is a function of the tissue stress sensed by said continuous diaphragm, said tissue stress resulting from blood pressure pulsations in said underlying artery.

41. The tissue contact stress sensing apparatus of claim 40, wherein said diaphragm is comprised of single crystal silicon.

42. The tissue contact stress sensing apparatus of claim 40, wherein said wafer has a longitudinal trough therein and wherein the bottom of said trough forms one side of said diaphragm and wherein said longitudinal trough has a cross-sectional profile generally resembling a tetragonal-pyramidal geometry.

43. The tissue contact stress sensing apparatus of claim 42, wherein said trough has a generally planar bottom and wherein said trough bottom is coated with a material for reflecting the electromagnetic radiation irradiated by said integrated circuit means.

44. The tissue contact stress sensing apparatus of claim 43, wherein said trough bottom is generally 6.5.times.10.sup.-6 meters thick.

45. The tissue contact stress sensing apparatus of claim 43, wherein said trough bottom is coated with metal and wherein said metal is selected from the group consisting of gold or aluminum and wherein said metal coating is generally 600 angstroms thick.

46. The tissue contact stress sensing apparatus of claim 40, wherein said diaphragm has an effective stiffness of generally fifty times greater than that typical of said tissue over laying said artery.

47. The tissue contact stress sensing apparatus of claim 40, wherein said continuous diaphragm has a strain rate of generally 0.24 micro-inch/mmHg.

48. The tissue contact stress sensing apparatus of claim 40, wherein said diaphragm has two opposing longitudinal sides spaced generally 0.020 inches apart.

49. The tissue contact stress sensing apparatus of claim 40, wherein said wafer has a generally rectangular face for placing against said tissue overlying said artery, said face having a first and second set of opposing parallel sides, wherein said first set of opposing sides of said rectangular face are generally 0.577 inches apart, and wherein said diaphragm is bounded by at least two generally parallel sides spaced generally 0.425 inches apart and wherein said second set of opposing parallel sides of said face are generally 0.200 inches apart.

50. The tissue contact stress sensing apparatus of claim 40, further including a spacing element disposed between said continuous diaphragm and said integrated circuit means for providing alignment and positioning of said integrated circuit means in relation to said diaphragm and wherein said spacing element has a thermal expansion coefficient substantially similar to that of said continuous diaphragm and wherein said spacing element is comprised of silicon nitride.

51. The tissue contact stress sensing apparatus of claim 40, wherein said integrated circuit means is comprised of a plurality of emitters for irradiating said diaphragm with electromagnetic radiation, and a plurality of detectors each having an output, each detector receiving a portion of said electromagnetic radiation and transducing said received radiation into a respectively associated electric current output signal.

52. The tissue contact stress sensing apparatus of claim 51, wherein said continuous diaphragm is generally planar having a rectangular boundary, said boundary having an opposing pair of long sides and an opposing pair of short sides and wherein said plurality of emitters are arranged generally in a row which is generally parallel to and spaced apart from one of said long sides of said diaphragm, and wherein said detectors are arranged generally in a row separate from said row of emitters, and generally parallel to and spaced apart from one of said long sides of said diaphragm.

53. The tissue contact stress sensing apparatus of claim 52, wherein said emitters are generally equally spaced apart and said detectors are generally equally spaced apart, and wherein said row of emitters is juxtaposed to said row of detectors such that any two adjacent detectors in said row of detectors lie equidistant from at least one common emitter in said row of emitters.

54. The tissue contact stress sensing apparatus of claim 51, wherein each emitter in said plurality of emitters irradiates electromagnetic radiation in a beam field which when projected onto said diaphragm, overlaps the projected beam field of its closest adjacent neighbors.

55. The tissue contact stress sensing apparatus of claim 51, wherein each emitter radiates electromagnetic energy which follows a Lambertian pattern about an axis normal to an emitting surface of said emitter and wherein said electromagnetic radiation is selected from the group consisting of visible, infrared and ultraviolet light.

56. The tissue contact stress sensing apparatus of claim 51, further comprising converter means coupled to the outputs of said detectors for converting said electric current signal output by each detector into and electric voltage signal and further comprising multiplexing means and central processing means said multiplexing means coupled to said converter means for selecting one of said converted electronic signals and transferring said signal to said central processing means for processing.

57. The tissue contact stress sensing apparatus of claim 40, wherein said integrated circuit means further includes a referencing portion for irradiating electromagnetic radiation onto and detecting electromagnetic radiation reflected from a surface of said nonresponsive portion of said silicon wafer, said nonresponsive wafer surface spaced apart from said diaphragm and wherein said integrated circuit means and said referencing portion of said integrated circuit means are fabricated from substantially the same materials and reside in close proximity to each other and wherein said electromagnetic radiation is selected from the group consisting of visible, infrared and ultraviolet light.

TECHNICAL FIELD

The present invention generally relates to a pressure measurement system, and more particularly relates to a system for noninvasively determining the blood pressure of a patient by detecting the surface stress of tissue above an arterial vessel.

BACKGROUND OF THE INVENTION

Systems for determining the intra-arterial blood pressure of a patient can be subdivided into two main groups-those which invade the arterial wall to directly access blood pressure and those which use non invasive techniques. For a long period of time, the most accurate blood pressure measurements were achievable only by use of invasive methods. One such common method involved use of a fluid filled catheter inserted into the patient's artery.

While invasive methods provide for accurate blood pressure measurements, the risk of infection and potential for complications, in many cases, outweigh the advantages of using invasive methods. Because of the risk of complication associated with invasive methods, a noninvasive method, known as the Korotkoff method is widely used. The Korotkoff method is known as an auscultatory method because it uses the characteristic sound made as the blood flows through the artery to denote the high and low blood pressure points. Although the Korotkoff method is noninvasive, it only provides a measurement of the highest blood pressure (systolic) and the lowest blood pressure (diastolic) along the continuous pressure wave form. While systolic and diastolic pressure are often sufficient for accurate diagnosis, there are many applications in which it is desirable to use the entire curve of the blood pressure wave form. In these applications, the Korotkoff method simply is incapable of providing satisfactory information. In addition to this limitation of the Korotkoff method, it necessitates the temporary occlusion of the artery in which blood pressure is being monitored. While arterial occlusion is not prohibited in many applications, there are occasions where the patient's blood pressure must be monitored continuously (such as when undergoing surgery) and accordingly, prohibiting blood flow, even on a temporary basis, is undesirable or unacceptable. Other problems associated with the Korotkoff method include the fact that the cuff must be properly sized with respect to the patient and the detrimental affects of respiration and acoustic noise on overall measurement accuracy.

Because of the above mentioned risks involved with invasive blood pressure measurement, and the shortcomings of the Korotkoff method, extensive investigation has been conducted in the area of continuous, noninvasive blood pressure monitoring and recording methods. Many of these noninvasive techniques make use of tonometric principles which center around the fact that as blood flows through the arterial vessel, forces are transmitted through the artery wall and through the surrounding arterial tissue and are accessible for monitoring. Because the tonometric method of determining blood pressure is noninvasive, it is used without the risks associated with invasive techniques. Furthermore, since it does not suffer from the limitations of the auscultatory method, it has the capability of reproducing the entire blood pressure wave form, as opposed to the limited systolic and diastolic pressure points provided by the Korotkoff method.

In several of the prior art arterial tonometers, a row of individual transducer elements, such as strain gauges or the like, are placed in direct contact with the tissue which overlays an arterial vessel from which blood pressure is to be measured. As the blood pressure within the arterial vessel increases and decreases the vessel wall expands and contracts thereby transmitting forces through the overlying tissue and onto the row of transducer elements. Although the individual elements are dimensionally sized so that several are required to cover the entire diameter of the underlying arterial vessel, their discrete character prevents reconstructing a true continuous contour of the tissue stresses which occur across the entire row of elements.

It has also been found that many prior art tonometry sensors are cumbersome, difficult to administer and uncomfortable to wear for any long period of time.

Thus, it is desirable to provide a noninvasive tonometry system for determining the blood pressure in an arterial vessel by measuring the stress of the tissue overlaying the arterial vessel.

Still further, it is desirable to have a system which is capable of accurately reconstructing a continuous stress contour across the diameter of an artery of interest.

It is also desirable to have a system which automatically compensates for errors introduced into the tissue stress signal which result from temperature, aging or other factors which influence the tissue stress sensor.

Additionally, it is desirable to have a miniaturized sensor which can be easily administered and comfortably worn for long periods of time.

SUMMARY OF THE INVENTION

In light of the foregoing objects, the present invention provides a tissue contact stress sensor for use in a system for noninvasively determining the intra-arterial blood pressure of a patient. The tissue contact stress sensor comprises a continuous diaphragm for placing against a patient's tissue which covers an underlying artery. The diaphragm is adapted to be deformed in response to stresses in the tissue created by the arterial blood pressure within the underlying artery. A semiconductor assembly is placed in close proximity to and spaced apart from the continuous diaphragm. The semiconductor assembly irradiates the diaphragm with electromagnetic radiation and receives a portion of the electromagnetic radiation which is reflected from the continuous diaphragm. The quantity of electromagnetic radiation which is received by the semiconductor assembly is a function of the stress experienced by the tissue overlaying the artery. Under controlled conditions, intra-arterial blood pressure can be determined by measuring the stress of the tissue overlaying the artery. Because the semiconductor assembly performs both the irradiating and receiving function, it allows the tissue contact stress sensor to be miniaturized and allows the semiconductor assembly to be placed very close to the continuous diaphragm. In a preferred embodiment, the diaphragm is comprised of silicon and includes a silicon base having a longitudinal trough therein. The cross-sectional profile of the trough generally resembles a tetragonal-pyramidal geometry. The trough bottom is generally planar and is preferably coated with a material for reflecting the electromagnetic radiation irradiated by the semiconductor assembly.

The sensing apparatus preferably includes a spacing element disposed between the diaphragm and the semiconductor assembly for providing fixed alignment and positioning of the semiconductor assembly in relation to the diaphragm. Preferably the spacing element has a thermal expansion coefficient substantially similar to the continuous diaphragm. This arrangement ensures that thermal stresses will be minimized between the two elements.

In a preferred embodiment, the semiconductor assembly is comprised of a plurality of emitters for irradiating the diaphragm with electromagnetic radiation and a plurality of detectors each having an output and each detector receiving a portion of the electromagnetic radiation which is reflected from the diaphragm and transducing the received radiation into a respectively associated electronic output signal. The continuous diaphragm is preferably generally planar having a rectangular boundary which has an opposing pair of long sides and an opposing pair of short sides. The plurality of emitters are preferably arranged generally in a row which is generally parallel to and spaced apart from one of the long sides of the diaphragm and the detectors are preferably arranged generally in a row spaced from the row of emitters and generally parallel to and spaced apart from one of the long sides of the diaphragm. Each emitter in the row of emitters is generally equally spaced apart from its adjacent neighbors and each detector in the row of detectors is generally equally spaced apart from its adjacent neighbors. The row of emitters is juxtaposed to the row of detectors such that any two adjacent detectors in the row of detectors lie equidistant from at least one common emitter in the row of emitters.

Each emitter preferably irradiates electromagnetic radiation in a beam field which when projected onto the diaphragm overlaps the projected beam field of its closest adjacent neighbors. Preferably, the electromagnetic radiation is selected from the group consisting of visible, infrared and ultraviolet light.

A portion of the semiconductor assembly (referencing portion) is used for irradiating electromagnetic radiation onto and detecting electromagnetic radiation reflected from a reference surface which does not move (i.e. is nonresponsive) with respect to tissue stress applied to the diaphragm. The reference surface is spaced apart from the diaphragm.

A current to voltage converter is respectively associated with the output of each detector for converting the current signal output by each respective detector to a voltage signal. A multiplexer is connected to the output of the converters for multiplexing their output to a central processing means wherein the data contained in the output signal of the converters is conditioned and processed.

Still further, the present invention provides a tissue contact stress sensor which generates a tissue contact stress signal indicative of the intra-arterial blood pressure of an arterial vessel and a correction signal for compensating for errors in the tissue contact stress signal caused by temperature drift and aging of the sensor. The system comprises a continuous diaphragm for placing against a patient's tissue which covers an underlying artery. The diaphragm is adapted to deform in response to stresses in the tissue overlaying the artery.

A semiconductor assembly is spaced apart from and placed in close proximity to the continuous diaphragm for directly irradiating the diaphragm with electromagnetic radiation and receiving a portion of the electromagnetic radiation irradiating from the continuous diaphragm. The semiconductor assembly converts the received radiation into a tissue contact stress signal (first signal) which represents blood pressure data whereby the quantity of electromagnetic radiation which is received by the semiconductor assembly is a function of the intra-arterial blood pressure. A spacing structure is coupled to the diaphragm and the semiconductor assembly thereby fixing the separation between the semiconductor assembly and the diaphragm. A referencing portion of the semiconductor assembly is spaced apart from and placed in close proximity to a wafer for directly irradiating a nonresponsive, reflective portion of the underside of the wafer with electromagnetic radiation and receiving a portion of the electromagnetic radiation reflected from the nonresponsive portion of the wafer. The referencing portion of the semiconductor assembly converts the received radiation into a second signal which represents reference data which is indicative of at least one reference factor whereby any change in the radiation received by the referencing portion of the semiconductor assembly is attributed to a change in at least one reference factor. The second signal is combined with the first signal in a manner which minimizes the dependence of the tissue contact stress data on the reference factors. The reference factors preferably include temperature of the sensor and effects due to sensor aging. The semiconductor assembly and the referencing portion of the semiconductor assembly are preferably fabricated from substantially the same materials and at substantially the same time and reside in substantially the same environment. This ensures that they track each other's age, temperature and other commonly shared environmental factors.

Still further the present invention provides a method for correcting errors in the output signal of a tissue contact stress sensor which are caused by the effects of aging and environmental factors on the sensor. The tissue contact stress sensor is of the type having an element for transducing blood pressure of a patient into a tissue stress signal. This method includes constructing a tissue contact stress sensor and a reference sensor from substantially identical materials at substantially the same time. The tissue contact stress sensor is adapted to be responsive to the tissue stress in the region of a superficial vessel and the reference sensor is adapted to be responsive to a fixed structural reference which does not vary with the tissue stress measured by the tissue contact stress sensor. The tissue contact stress sensor and a reference sensor are adapted to share the same environment so as to be equally influenced by aging, temperature and other environmental factors. Signals generated by the tissue contact stress sensor and a reference sensor are combined such that the errors caused by the effects of aging, temperature and other environmental factors are minimized.

Other advantages and meritorious features of the presentation will become more fully understood from the following description of the preferred embodiments, the appended claims and the drawings, a brief description of which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the system of the present invention as applied to the wrist of a patient.

FIG. 2 is a cross-sectional view of the system of the present invention taken substantially along lines 2--2 of FIG. 1.

FIG. 3 is an amplified view of the system of the present invention taken substantially within encircled portion 3 of FIG. 2.

FIG. 4 is a cross-sectional view of the tissue contact stress sensor of the present invention taken substantially along lines 4--4 of FIG. 3.

FIG. 5 is a cross-sectional view of the tissue contact stress sensor of the present invention taken substantially along lines 5--5 of FIG. 4.

FIG. 6 is a partially exploded view of the tissue contact stress sensor of the present invention.

FIGS. 7a and 7b are diagrammatic views of the emitter and detector portions of the semiconductor assembly of the present ivention.

FIG. 8 is an electronic block diagram of the tissue contact stress sensor of the present invention.

FIG. 9 is a detailed schematic of the block diagram of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now referring to the drawing of FIG. 1, arterial tonometer 10 is placed about wrist 12 of a patient for determining the patient's blood pressure. Arterial tonometer 10 measures the patient's intra-arterial blood pressure noninvasively by sensing the time varying surface tissue contact stresses in regions immediately over and in the vicinity of the artery of interest. In general, the artery to be measured must be superficial and overlaying relatively rigid structures. The arteries most commonly used are the radial artery in the wrist, the superficial temporal artery in the forehead the and the dorsalis pedis artery in the foot. For purposes of this disclosure, the radial artery in the wrist will be used as the main artery of discussion; however, the system of the present invention is equally applicable to any superficial artery which overlays a relatively rigid structure.

Now referring to the drawing of FIG. 2, arterial tonometer 10 comprises clamp body 14, rack 16, push arm 18 and sensor 20. Rack 16 is driven by motor 13 through pinion 11. Clamp 14 can take on any number of configurations and is shown here in a generic form. Motor 13 is adapted to rotate thereby rotating pinion 11 and moving rack 16. Any movement of rack 16 moves sensor 20 via push arm 18. When motor 13 is rotated in the appropriate direction, sensor 20 is forced against tissue 24 which overlays radial artery 26. The displacement caused by sensor 20 is adjusted to a level which properly applanates radial artery 26 without causing artery 26 to occlude.

An important element of the present invention is sensor 20 and its surrounding structure. Because sensor 20 is used to compress or applanate radial artery 26 during blood pressure measurement as well as measure the contact stress in tissue 24, the geometry of sensor 20 and its surrounding structure are vital to the proper conduction of stresses from radial artery 26 to tissue surface 28. A detailed discussion of sensor 20 and its associated structure now follows.

Now referring to the drawing of FIG. 3, sensor 20 includes wafer 30 which has a nonresponsive portion 32 and a responsive portion (diaphragm) 34. Nonresponsive portion 32 serves mainly to support and press responsive portion 34 upon tissue overlying radial artery 26. Under normal conditions when sensor 20 is not being applied to tissue 24, radial artery 26 has a generally rounded cross-section as depicted at 26'. As wafer 30 of sensor 20 is pressed upon tissue 24, radial artery 26' begins to applanate or flatten along its top surface 36, causing responsive portion 34 of wafer 30 to deflect slightly inward 38. As the blood pressure within radial artery 26 changes (i.e. pulsates), stress is created in tissue 24 which disturbs the equilibrium between responsive portion 34 of wafer 30 and top surface 28 of overlying tissue 24. This disturbance in equilibrium causes movement between diaphragm 34 and surface 24. Such movement exists until a new equilibrium is established. The ability of diaphragm 34 to move and assume a unique displacement position for a given blood pressure within radial artery 26 forms the fundamental mechanism whereby sensor 20 is able to transduce the intra-arterial pressure of radial artery 26. The details of sensor 20 will now be fully discussed.

Now referring to the drawing of FIG. 4, tissue contact stress sensor 20 is comprised of sensor head 40 and sensor base portion 42. Sensor head 40 comprises the transducer portion of sensor 20 and sensor base portion 42 includes electronic circuitry and other mechanical support structure necessary for properly operati